Neurotoxins with enhanced target specificity

ABSTRACT

Modified neurotoxins that contain protease cleavage sites susceptible uniquely to proteases present in certain tissues are described. The toxins can be selectively activated by proteases in muscle or selectively inactivated by proteases in blood.

This application is a divisional and claims priority pursuant to 35 U.S.C. §120 to U.S. patent application Ser. No. 09/726,949, filed Nov. 29, 2000, which is hereby incorporated by reference in its entirety.

TECHNICAL FIELD

The invention is directed to modified neurotoxins which are deactivated in tissues where toxic activity is undesirable and neurotoxins that are activated at desired targets. More specifically, the invention concerns insertion of cleavage sites in non-critical regions of the toxins which are susceptible to protease activity in non-target tissues and to single chain forms which are activated by proteases in the target.

BACKGROUND ART

Structure and Function

Neurotoxins, such as those obtained from Clostridium botulinum and Clostridium tetani, are highly potent and specific poisons of neural cells. Both the single known tetanus toxin and the multiplicity of known botulinum toxins comprise, in their activated forms, two peptide acid chains coupled through a disulfide link: a light chain (LC) of about 50 KDa and a heavy chain (HC) of about 100 KDa. The toxins are synthesized in vivo as single chains, which are not toxic. However, the toxin becomes active when the single chain is nicked in a post-translational modification to form the separate LC and HC (linked by S-S).

The tetanus and botulinum toxins have lethal doses in humans of between 0.1 ng and 1 ng per kilogram of body weight. They function by inhibiting neurotransmitter release in affected neurons. The tetanus neurotoxin (TeNT) acts mainly in the central nervous system, while botulinum neurotoxin (BoNT) acts at the neuromuscular junction and other cholinergic synapses in the peripheral nervous system. Both types act by inhibiting acetylcholine release from the axon of the affected neuron into the synapse, resulting in paralysis. The effect of intoxication on the affected neuron is long-lasting and until recently has been thought to be irreversible.

Only one form of tetanus neurotoxin is known; seven different immunologically distinct forms of botulinum neurotoxins termed BoNT/A through BoNT/G are known. While all of these types are produced by isolates of C. botulinum, two other species, C. baratii and C. butyricum also produce toxins similar to /F and /E, respectively.

Regardless of type, the molecular mechanism of intoxication appears to be similar. First, the toxin binds to the presynaptic membrane of the target neuron through a specific interaction between the heavy chains (HC) and a cell surface receptor; the receptor is thought to be different for each type of botulinum toxin and for TeNT. The carboxyl terminus of the HC appears to be important for targeting of the toxin to the cell surface.

In the second step, the toxin crosses the plasma membrane of the poisoned cell, is engulfed by the cell through receptor-mediated endocytosis, and an endosome containing the toxin is formed. The toxin then escapes the endosome into the cytoplasm of the cell. The escape is thought to be mediated by a conformational change brought about by the acidic environment within the endosome which is effected by a proton pump that decreases intraendosomal pH. At a pH of about 5.5 or lower, the sequence at the amino terminus of the heavy chain triggers this conformational change. The conformation shift exposes hydrophobic residues which permits the toxin to embed itself in the endosome membrane and then translocate into the cytosol.

Once in the cytosol, reduction of the disulfide bond joining the HC and LC takes place. The entire toxic activity of botulinum and tetanus toxins is contained in the LC; which is a zinc (Zn⁺⁺) endopeptidase that selectively cleaves “SNARE” proteins essential for recognition and docking of neurotransmitter-containing vesicles with the cytoplasmic surface of the plasma membrane, and fusion of the vesicles with the plasma membrane.

The “SNARE” proteins are of several forms which have differential responses to the various forms of toxin. TeNT, BoNT/B BoNT/D, BoNT/F, and BoNT/G cause degradation of synaptobrevin (also called vesicle-associated membrane protein (VAMP)), a synaptosomal membrane protein. Most of the cytosolic domain of VAMP extending from the surface of the synaptic vesicle is removed as a result of any one of these cleavage events. Each toxin (except TeNT and BoNT/B) specifically cleaves a different bond.

BoNT/A and /E selectively cleave the plasma membrane-associated protein SNAP-25; this protein is predominantly bound to and present on the cytosolic surface of the plasma membrane. BoNT/C cleaves syntaxin, an integral protein having most of its mass exposed to the cytosol. Syntaxin interacts with the calcium channels at presynaptic terminal active zones.

Both TeNT and BoNT are taken up at the neuromuscular junction. BoNT remains within peripheral neurons, and blocks release of the neurotransmitter acetylcholine from these cells. TeNT enters vesicles that move in a retrograde manner along the axon to the soma, and is discharged into the intersynaptic space between motor neurons and the inhibitory neurons of the spinal cord. At this point, TeNT binds receptors of the inhibitory neurons, is again internalized, and the light chain enters the cytosol to block the release of the inhibitory neurotransmitters 4-aminobutyric acid (GABA) and glycine from these cells.

Pharmaceutical Applications

Dilute preparations of BoNT have been used since 1981 as therapeutic agents in the treatment of patients having various spastic conditions, including strabismus (misalignment of the eye), bephlarospasm (involuntary eyelid closure) and hemifacial spasm. See e.g., Borodic, et al., Pharmacology and Histology of the Therapeutic Application of Botulinum Toxin in Therapy with Botulinum Toxin 119-157 (Jankovic J. & Hallett, eds. 1994), hereby incorporated by reference herein. The toxin preparations are delivered specifically and locally to the site of the neurons to be effected. BoNT/A is the most potent of the BoNT's, and the best characterized. Intramuscular injection of dilute preparations of BoNT/A has also been used effectively to treat spastic conditions due to brain injury, spinal cord injury, stroke, multiple sclerosis and cerebral palsy. The extent of paralysis depends on both the dose and volume delivered to the target site.

Clearly, it is desirable to confine the activity of the administered toxin to the target site. A number of strategies have been adopted, including, besides direct injection, implantation of a capsule pump or administration of a slow release gel. However, the success of such attempts has been far from complete. Because of the diffusion of the toxin from the site of administration, systemic problems, such as difficulty in swallowing have occurred. The reality of these undesired effects has limited the level of dosage which can be administered. For example, subjects needing treatment in both arms or both legs generally cannot be administered the toxin in both affected limbs simultaneously due to the side effects. It would thus be desirable to provide a form of the toxins which inherently acts specifically at its target site. The present invention provides such modified forms.

DISCLOSURE OF THE INVENTION

The invention provides botulism and tetanus toxins, including variants and derivatives thereof, which contain protease target sites in non-critical regions such that specificity of toxicity with regard to a particular target tissue is conferred. In general, the toxins are active at the neuromuscular junction and thus, should be toxic in muscle tissue; they may also be active at sites in the central nervous system. Systemic spread occurs mainly through the bloodstream, and it would thus be desirable to inactivate the toxin as soon as any molecules enter the bloodstream. By suitable choice of cleavage sites, susceptible to blood or muscle proteases respectively, the toxins can be provided in a form which will be inactivated in blood, activated in muscle, or both. The cleavage site must be provided in a region of the toxin where its presence does not disrupt the activity of the toxin, but where cleavage results in activation or inactivation as the case may be.

Thus, in one aspect, the invention is directed to a modified botulism or tetanus toxin wherein the modification comprises the inclusion of a cleavage site in a domain that must remain intact for activity and where the cleavage site is susceptible to cleavage by a protease that is present in effective levels only in a tissue where toxic activity is undesirable and where the cleavage site itself does not inactivate the toxin. In another aspect, the invention is directed to a modified botulism or tetanus toxin wherein the modification comprises a target cleavage site for a protease such that an inactive form of the toxin is activated. In this case, the protease specific for the cleavage site would be present in an effective amount only in tissues where toxicity is desired.

Still another aspect, the invention is directed to toxins which are modified to contain cleavage sites of both types. Thus, such a toxin would be specifically activated in muscle but deactivated once it was transported into the blood stream.

In still another aspect, the invention is directed to recombinant materials encoding the modified toxins and to methods to produce them. The invention is also directed to methods to treat conditions benefited by neurotoxin activity which comprises administering the neurotoxins of the invention or administering expression systems for their localized production.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the amino acid sequence of botulism A toxin purified from Clostridium botulinum strain Hall A where residues 437-438 which are cleaved during post-translational modification are adopted from the conserved sequence obtained by DNA sequences of strains NCTC2916 and 62A.

FIG. 2 is a graph showing the stability of botulism toxin A in human serum.

FIG. 3 is a graph showing the resistance of botulism toxin A to various proteases.

MODES OF CARRYING OUT THE INVENTION

The botulism and tetanus toxins which are modified in order to obtain the desirable properties associated with the invention include, in addition to the naturally occurring forms, variants and derivatives of these forms. Such variants include, for example, chimeric forms of these toxins in which a portion of the heavy chain or light chain of BoNT/A is replaced by the corresponding region of BoNT/E or BoNT/G or tetanus toxin. Various combinations can be envisioned. The toxin may have a heavy chain from one native toxin combined with light chain of another. In addition, variants may contain, in regions that are irrelevant to activity, 1-5 substitutions, preferably by amino acids which are similar in character—i.e., conservative amino acid substitutions. The variants may also contain deletions of 1-5, preferably 1-3, more preferably 1-2 amino acids in regions where the activity is substantially unaffected. Derivatives of these toxins include forms that may be lipidated, PEGylated, phosphorylated, or otherwise derivatized by additional covalently bonded groups, including the N-terminal acylated and C-terminal amidated forms. Thus, the toxins which are modified according to the present invention may include a range of variants and derivatives of the naturally occurring toxins. As used and defined herein, the terms “botulism toxin” and “tetanus toxin” refer not only to the native toxins, but also to the variants and derivatives as described above.

The invention is directed to modified forms of botulism or tetanus toxin (including variants and derivatives as defined above) which contain protease cleavage sites. In the aspect wherein the cleavage site will result in inactivation of the toxin, the site must be present in a region wherein the presence of the cleavage site itself does not destroy toxic activity. By “does not destroy toxic activity” is meant that the toxic activity of the modified form is at least 10%, preferably 25%, more preferably 50%, more preferably 75% and most preferably at least 90% of the unmodified form. Forms wherein the toxic activity is equal to or exceeds that of the toxin itself are also included. Such non-critical regions can be determined experimentally by assessing the resulting toxicity of the modified toxin using standard toxicity assays such as that described by Zhou, L., et al., Biochemistry (1995) 34:15175-15181, which describes an in vitro assay for the ability of light chain to cleave recombinantly produced SNAP-25. Other suitable assays commonly practiced include simple injection into mice to evaluate lethality. These assays are described, for example, by Maisey, E. A., et al., Eur J. Biochem. (1988) 177:683-691.

However, rational decisions about the location of such sites can be based on the known conformation of the toxins. The crystal structure of botulism toxin type A is described, for example by Lacy, D. B., et al., Nature Structural Biology (1998) 5:898-902. Based on the crystal structure, and other data obtained with respect to botulism toxin type A, certain features are apparent as will be described below. Because of the similarity of all of the botulism toxins and tetanus toxins, the description that is set forth below in detail with respect to type A is applicable as well to the remaining toxin types and to tetanus toxins. The primary amino acid sequences are similar in all cases, and the functions and mechanisms of action are similar as well. Briefly, it appears that the light chain is globular, containing a number of exposed loops of minimal secondary structure; the heavy chain comprises two globular regions and a paired double helix. An extension of the light chain comprising a “belt” circumscribes a portion of the complex. Suitable regions for locating a protease target site, wherein cleavage results in inactivation would be found between the binding and transport domains, between the two globules of the binding domain of the heavy chain, in the non-catalytic regions of the light chain, and in the belt region.

Other features of the protein are also known. Recent studies of the BoNT/A light chain have revealed certain features important for the activity and specificity of the toxin towards its target substrate, SNAP-25. Thus, studies by Zhou, et al., Biochemistry 34:15175-15181 (1995) have indicated that when the light chain amino acid residue His₂₂₇ is substituted with tyrosine, the resulting polypeptide is unable to cleave SNAP-25; Kurazono, et al., J. Biol. Chem. 14721-14729 (1992) performed studies in the presynaptic cholinergic neurons of the buccal ganglia of Aplysia californica using recombinant BoNT/A light chain that indicated that the removal of 8 N-terminal or 32 C-terminal residues did not abolish toxicity, but that removal of 10 N-terminal or 57 C-terminal residues abolished toxicity in this system. Most recently, the crystal structure of the entire BoNT/A holotoxin has been solved; the active site is indicated as involving the participation of His₂₂₂, Glu₂₂₃, His₂₂₆, Glu₂₆₁ and Tyr₃₆₅. Lacy, et al., supra. (These residues correspond to His₂₂₃, Glu₂₂₄, His₂₂₇, Glu₂₆₂ and Tyr₃₆₆ of the BoNT/A L chain of Kurazono et al., supra.) Interestingly, an alignment of BoNT/A through E and TeNT light chains reveals that every such chain invariably has these residues in positions analogous to BoNT/A. Kurazono, et al., supra.

The catalytic domain of BoNT/A is very specific for the C-terminus of SNAP-25 and appears to require a minimum of 17 SNAP-25 amino acids for cleavage to occur. The catalytic site resembles a pocket; when the light chain is linked to the heavy chain via the disulfide bond between Cys₄₂₉ and Cys₄₅₃, the translocation domain of the heavy chain appears to block access to the catalytic pocket until the light chain gains entry to the cytosol. When the disulfide bond is then reduced, the catalytic pocket is “opened” and the light chain is fully active. As described above, VAMP and syntaxin are cleaved by BoNT/B, D, F, G and TeNT, and BoNT/C₁, respectively, while SNAP-25 is cleaved by BoNT/A and E.

While the presence of the cleavage site itself must permit the toxic activity to be retained, actual cleavage at the site must result in inactivation. By inactivation in this context is meant that the toxin retains only 50% of the toxicity, preferably only 25% of the toxicity, more preferably only 10% of the toxicity, more preferably only 1% of the toxicity of the uncleaved form. Thus, while the position of the site should be in a non-critical region with respect to the site itself, cleavage at that site must have a substantial effect.

For the design of a botulism toxin which can be inactivated by blood, protease sites which are recognized by proteases relatively uniquely found in the bloodstream are desirable. Among these proteases are those set forth below in Table 1 which also describes their recognition sites. TABLE 1 Proteases Present in Blood Blood protease Substrate Specificity Thrombin P4-P3-P-R/K*P1′-P2′ P3/P4 hydrophobic; P1′/P2′ non-acidic P2-R/K*P1′ P2 or P1′ are G Coagulation Factor Xa I-E-G-R* (SEQ ID NO: 13) I-D-G-R* (SEQ ID NO: 14) Coagulation Factor XIa R* Coagulation Factor XIIa R* Coagulation Factor IXa R* Coagulation Factor VIIa R/K* Kallikrein R/K* Protein C R* MBP-associated serine R* protease Oxytocinase N-terminal C* Lysine carboxypeptidase C-terminal R/K* *indicates the peptide bond this protease will cleave.

As is clear, coagulation factors XIa, XIIa, IXa and VIIa as well as kallikrein, protein C, MBP-associated serine protease, oxytocinase and lysine carboxypeptidase have relatively nonspecific target sites, while coagulation factors Xa and thrombin provide the opportunity for more specificity.

In designing a thrombin or coagulation factor Xa site into a botulism toxin, the location of the inserted site is, as described above, such that the presence of the site will not interfere with activity of the toxin, but cleavage at the site will destroy or vastly inhibit the activity of the toxin. In general, the early steps of the action can be targeted by placing the site into the receptor binding region or the internalization region in the heavy chain, but away from the functional domains within these regions. Insertion sites in the heavy chain receptor binding domain should be away from receptor binding grooves and in all cases the sites should be selected so as to be on the surface of the protein so that blood proteases can freely access them.

Table 2 indicates some examples of site modifications for cleavage by thrombin in the botulism toxin A amino acid sequence set forth in FIG. 1. In the table, the underline indicates a thrombin recognition site, the residues in bold are the additional amino acids; residues which were present in the native sequence but are eliminated when the site is inserted are in parentheses. An asterisk indicates the peptide bond the thrombin will cleave. TABLE 2 Thrombin Site Insertion Amino acid residue Location Example of thrombin site insertion 930-AIVYNS-935 (SEQ ID NO: 2) H_(C) -A-I-R*G(VY)-N-S- (SEQ ID NO: 3) -A-I-P-R*(VY)-N-S- (SEQ ID NO: 4) -A-I-P-R*V-Y-N-S- (SEQ ID NO: 5) 1060-RDTH-1063 (SEQ ID NO: 6) H_(C) -G-R*D-T-H- (SEQ ID NO: 7) 1136-KGPRGSVMT-1144 (SEQ ID NO: 8) H_(C) -I (K)-G-P-R*G-S-V-M-T- (SEQ ID NO: 9) 1165-ASGNKDN-1171 (SEQ ID NO: 10) H_(C) -A-S-G-G(N)-K*D-N- (SEQ ID NO: 11) -A-L(S)-G-P(N)-K*G(D)-N- (SEQ ID NO: 12)

The locations of the sites proposed are also set forth in FIG. 1.

Thus, for the inactivating cleavage, the protease should be one present in high levels in blood. A suitable protease in this regard is thrombin, which, as shown below, occurs in blood in levels sufficient to deactivate the modified form of the toxins herein. By “effective” level of the protease is meant a concentration which is able to inactivate at least 50%, preferably 75%, more preferably 90% or greater of the toxin which enters the bloodstream at clinically suitable levels of dosage.

In general, the dosage levels for botulism toxins are on the order of nanogram levels of concentration and thus are not expected to require higher concentrations of protease.

With respect to forms of toxins which are selectively activated by muscle, preferably these comprise the single chain forms of the toxin precursor. Such forms, which are resistant to proteolytic activity by the Clostridium itself, have been designed for recombinant production in other organisms such as E. coli. Indeed, in some cases, the Clostridium produces mostly the uncleaved single chain form.

Single chain forms with a proteolytic site in the interchain loop region for cleavage by enzymes applied in vitro is described in application No. 60/150,710 filed 25 Oct. 1999, and incorporated by reference. As described in this application, a single chain form of tetanus or botulism toxin is constructed by ligating the nucleotide sequences encoding light and heavy chain through a linker region corresponding, for example, to residue 437-448 of FIG. 1. In the case of the present invention, this linker region is provided with a target site for a protease which is specifically present at sufficient concentrations in muscle to effect cleavage at this site.

As the production of the single chain form involves genetic manipulation, (as does the construction of the form which is subject to inactivation in blood) mixing and matching of various regions of the seven botulism toxins and the tetanus toxin is well within the skill of the artisan. Thus, a botulism A light chain might be used in combination with botulism toxin B heavy chain or with a tetanus heavy chain transmembrane region and a botulism G receptor recognition region. The particular toxin exemplified as a single chain product in the above-referenced provisional application is a single chain tetanus toxin, but clearly other single chain toxins with appropriate protease sites could be readily engineered. Thus, in the present invention, the nature of the protease cleavage site will be altered so as to be susceptible to muscle proteases.

The substrate neurotoxins which can be modified according to the method of the invention are those natively occurring as described in the Background section above, or can be themselves modified such as those described in PCT publications WO95/32738, WO96/33273, WO98/07864 and WO99/17806, all incorporated herein by reference. As stated above, the selectively activated and inactivated toxins of the invention have the further advantage that heavy chain and light chain components of the neurotoxins can be readily mixed and matched, and prepared as chimeras.

The following examples are intended to illustrate but not to limit the invention.

EXAMPLE 1 Thrombin Activity in Blood

The substrate for assay of thrombin proteolytic activity is the GST-SNAP-25 (a.a.128-197) fusion protein with a thrombin cleavage site inserted between the two proteins. The substrate was added in a solution containing 10 μg/1.55 μl to 8.45 μl of PBS or blood preparations to make the final volume at 10 μl. The mixtures were incubated at 37° C. One μl (containing 1 μg of the substrate) used in each treatment was mixed with 1 μl of 2× sample buffer with DTT and boiled for 5 minutes before loading to 18% SDS-PAGE gel. The gel was then blotted and probed with Ab 197, which recognizes the C-terminal end of the cleaved or uncleaved SNAP-25₁₂₈₋₁₉₇.

Results showed that the substrate was cleaved by serum and at a much lower level, by whole blood. No cleavage product was detected by incubating with plasma. The lack of thrombin activity in the plasma suggests the majority of the enzyme is probably still in its proenzyme form (prothrombin). Prothrombin is activated to thrombin as a result of a cascade of proteolytic enzymes that initiate blood coagulation, and thus thrombin is present in the serum because the clotting has been triggered, but not in plasma where the clotting was prohibited by adding citrate. the trace amount of thrombin cleavage product seen in the sample incubated with citrated whole blood indicates that a low level of clotting pathway activation has occurred.

As typically these toxins are injected at very low (nanogram) levels, the thrombin levels in blood, even at the low levels of activation shown may be sufficient to effect at least sufficient cleavage substantially to inactivate the toxin. In addition, the trauma caused by injection of the toxin may trigger activation of prothrombin to elevate these levels.

EXAMPLE 2 Stability of Native BoNT/A with Respect to Proteases

To test the stability of unmodified BoNT/A in human serum, BoNT/A was added at 10 μg/ml of human serum and incubated for the time specified. The activity of the toxin was measured by the absorbance at 450 nm in the SNAP-25 assay described in co-pending application Ser. No. 09/164,692, filed 25 Aug. 2000, claiming priority to provisional application Ser. No. 60/150,710 filed 25 Aug. 1999, both incorporated herein by reference. The absorbance value was compared at the dilution of 1000 μg toxin per ml. As shown in FIG. 2, BoNT/A is fairly stable in human serum for up to 4 days and still retained 50% of the activity after 5 days in the serum. Similar results were found when incubating pure BoNT/A or complex with whole blood or plasma and the toxins were stable for up to 48 hours tested. These results suggest BoNT/A is stable in the blood without immediate breakdown by plasma protease and therefore will potentially circulate to other muscles and organs and cause a systemic effect unless inactivated by the invention method.

The resistance of BoNT/A to various proteases. BoNT/A was incubated overnight with excess purified restricted proteases at room temperature and then the activity was analyzed by the SNAP-25 assay such as that referenced above. Factor Xa and thrombin are plasma proteases; enterokinase is present in the duodenal secretions into the GI tract. Activities are compared at 100 pg/ml dilution of each toxin reaction. The results are shown in FIG. 3. The toxin is resistant to these proteases. Although the toxin incubated with thrombin lost ⅓ of its activity, it may not be very sensitive to thrombin degradation since an excess of enzyme was used for a long period of time. SDS-PAGE gel analysis of these treated toxins showed no cleavage. 

1. A nucleic acid molecule which comprises a nucleotide sequence encoding a modified botulinum A neurotoxin comprising: a) a binding domain comprising a first heavy chain portion able to interact with a surface receptor present in a target cell, wherein said first heavy chain portion comprises a mammalian blood protease cleavage site, said mammalian blood protease cleavage site comprising a modification in one or more regions selected from the group consisting of amino acids 930-935 of SEQ ID NO: 1, amino acids 1060-1063 of SEQ ID NO: 1, amino acids 1136-1144 of SEQ ID NO: 1 and amino acids 1165-1171 of SEQ ID NO: 1; b) a translocation domain comprising a second heavy chain portion able to mediate the escape of a light chain portion of said neurotoxin from an endosome to the cytoplasm of said target cell; and c) an enzymatic domain comprising said light chain portion able to cleave a SNARE-protein present in said target cell; wherein said modified botulinum A neurotoxin is able to interact with a surface receptor of said target cell, able to mediate endosomal escape into the cytoplasm of said target cell and able to cleave a SNARE protein present in said target cell; and whereupon said modified botulinum A neurotoxin's ability to interact with a surface receptor of said target cell is inactivated upon cleavage of said mammalian blood protease cleavage site.
 2. A nucleic acid molecule which comprises a nucleotide sequence encoding a modified botulinum A neurotoxin comprising: a) a binding domain comprising a first heavy chain portion able to interact with a surface receptor present in a target cell, wherein said first heavy chain portion comprises a mammalian blood protease cleavage site, said mammalian blood protease cleavage site comprising a modification in one or more regions selected from the group consisting of amino acids 930-935 of SEQ ID NO: 1, amino acids 1060-1063 of SEQ ID NO: 1, amino acids 1136-1145 of SEQ ID NO: 1 and amino acids 1165-1171 of SEQ ID NO: 1; b) a translocation domain comprising a second heavy chain portion able to mediate the escape of a light chain portion of said neurotoxin from an endosome to the cytoplasm of said target cell; and c) an enzymatic domain comprising said light chain portion able to cleave a SNARE-protein present in said target cell, wherein said light chain portion comprises a mammalian muscle protease cleavage site, said mammalian muscle protease cleavage site comprising a modification in amino acids 430-452 of SEQ ID NO: 1; wherein said modified botulinum A neurotoxin is able to interact with a surface receptor of said target cell, able to mediate endosomal escape into the cytoplasm of said target cell and able to cleave a SNARE protein present in said target cell; and whereupon said modified botulinum A neurotoxin's ability to interact with a surface receptor of said target cell is inactivated upon cleavage of said mammalian blood protease cleavage site; and whereupon said modified botulinum A neurotoxin's ability to cleave a SNARE protein present in a target cell is activated upon cleavage of said mammalian muscle protease cleavage site.
 3. The nucleic acid molecule according to either claim 1 or claim 2, wherein said mammalian blood protease cleavage site is cleaved by a mammalian blood protease selected from the group consisting of Thrombin, Coagulation Factor VIIa, Coagulation Factor IXa, Coagulation Factor Xa, Coagulation Factor XIa, Coagulation Factor XIIa, Kallikrein, Protein C and MBP-associated serine protease.
 4. The nucleic acid molecule according to either claim 1 or claim 2, wherein said modification of amino acids 930-935 of SEQ ID NO: 1 is selected from the group consisting of SEQ ID NO: 3, SEQ ID NO: 4 and SEQ ID NO:
 5. 5. The nucleic acid molecule according to either claim 1 or claim 2, wherein said modification of amino acids 1060-1063 of SEQ ID NO: 1 is SEQ ID NO:
 7. 6. The nucleic acid molecule according to either claim 1 or claim 2, wherein said modification of amino acids 1136-1145 of SEQ ID NO: 1 is SEQ ID NO:
 9. 7. The nucleic acid molecule according to either claim 1 or claim 2, wherein said modification of amino acids 1165-1171 of SEQ ID NO: 1 is selected from the group consisting of SEQ ID NO: 11 and SEQ ID NO:
 12. 8. An expression system which comprises the nucleotide sequence of according to any one of claims 1-7 operably linked to sequences for effecting its expression.
 9. A recombinant host cell modified to contain the expression system of claim
 8. 10. A method to produce a modified botulinum A neurotoxin, the method comprising the steps of culturing the host cell of claim 9 under conditions wherein said modified botulinum A neurotoxin is produced and optionally recovering the modified botulinum A neurotoxin from the culture.
 11. A nucleic acid molecule which comprises a nucleotide sequence encoding a modified botulinum A neurotoxin comprising a blood protease cleavage site, wherein said blood protease cleavage site comprises a modification in one or more regions selected from the group consisting of amino acids 930-935 of SEQ ID NO: 1, amino acids 1060-1063 of SEQ ID NO: 1, amino acids 1136-1145 of SEQ ID NO: 1 and amino acids 1165-1171 of SEQ ID NO:
 1. 12. A nucleic acid molecule which comprises a nucleotide sequence encoding a modified botulinum A neurotoxin comprising a blood protease cleavage site and a muscle protease cleavage site, wherein said blood protease cleavage site comprises a modification in one or more regions selected from the group consisting of amino acids 930-935 of SEQ ID NO: 1, amino acids 1060-1063 of SEQ ID NO: 1, amino acids 1136-1145 of SEQ ID NO: 1 and amino acids 1165-1171 of SEQ ID NO: 1; and wherein said muscle protease cleavage site comprises a modification in amino acids 430-452 of SEQ ID NO:
 1. 13. The nucleic acid molecule according to either claim 11 or claim 12, wherein said mammalian blood protease cleavage site is cleaved by a mammalian blood protease selected from the group consisting of Thrombin, Coagulation Factor VIIa, Coagulation Factor IXa, Coagulation Factor Xa, Coagulation Factor XIa, Coagulation Factor XIIa, Kallikrein, Protein C and MBP-associated serine protease.
 14. The nucleic acid molecule according to either claim 11 or claim 12, wherein said modification of amino acids 930-935 of SEQ ID NO: 1 is selected from the group consisting of SEQ ID NO: 3, SEQ ID NO: 4 and SEQ ID NO:
 5. 15. The nucleic acid molecule according to either claim 11 or claim 12, wherein said modification of amino acids 1060-1063 of SEQ ID NO: 1 is SEQ ID NO:
 7. 16. The nucleic acid molecule according to either claim 11 or claim 12, wherein said modification of amino acids 1136-1145 of SEQ ID NO: 1 is SEQ ID NO:
 9. 17. The nucleic acid molecule according to either claim 11 or claim 12, wherein said modification of amino acids 1165-1171 of SEQ ID NO: 1 is selected from the group consisting of SEQ ID NO: 11 and SEQ ID NO:
 12. 18. An expression system which comprises the nucleotide sequence of according to any one of claims 11-17 operably linked to sequences for effecting its expression.
 19. A recombinant host cell modified to contain the expression system of claim
 18. 20. A method to produce a modified botulinum A neurotoxin, the method comprising the steps of culturing the host cell of claim 19 under conditions wherein said modified botulinum A neurotoxin is produced and optionally recovering the modified botulinum A neurotoxin from the culture. 